Sounds like you answered your own question by showing how impossible it is to answer with a timeline.
Brake use is very dependent on a vast variety of conditions and driving experience.
Many drivers, myself included, use the trolley brakes and engine brake more than the tractor brakes. By not being in a hurry in city traffic, it is possible to rarely have to use the brakes at all if lights are timed properly and engine brakes are used to limit coming to a full stop at a light. In some metropolitan areas, it is ‘illegal’ to use the engine retarder but, if your tractor has good baffles, it’s not even noticed.
In the flatlands I’m sure, the brakes can last nearly forever if you’re doing drop and hooks (using trailers as braking devices). Driving the Northeast or Rockies would change that guesstimate.
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